Diamond marking

ABSTRACT

An information mark invisible to the naked eye is applied to the polished facet of a diamond gemstone by coating the diamond gemstone surface with an electrically conductive layer so as to prevent the diamond becoming charged, forming the mark with a focused ion beam, and cleaning the diamond surface with a powerful oxidizing agent to reveal a mark having an appropriate depth, which does not detrimentally affect the clarity or color grade of the diamond.

BACKGROUND OF THE INVENTION

The present invention relates to a method of marking a surface of adiamond or gemstone. The mark may be any mark, but the invention isparticularly though not exclusively directed to applying an informationmark to the diamond or gemstone. The diamond may be for instance anindustrial diamond such as a wire-drawing die-or diamond opticalcomponent, though the invention is of particular interest in markinggemstone diamonds, for instance for applying a mark which is invisibleto the naked eye or invisible to the eye using a ×10 loupe, when themark can be applied to a polished facet of the gemstone withoutdetracting from its clarity or color grade. When a loupe is used, thevisibility is assessed under the internationally accepted conditions forclarity grading, i.e. using a 10× magnifying achromatic, aplanatic loupeunder normal light, this being a white diffuse light, not a spot light.The marks can be used to uniquely identify the gemstone by a serialnumber or as a brand or quality mark. In general, the mark should becapable of being viewed under suitable magnification and viewingconditions, and, if applied to a gemstone, should not detract from thevalue or appearance of the stone and should preferably not exhibitblackening.

There is a detailed description of the nature of the marks that can beapplied in WO 97/03846, which is incorporated herein by reference and inwhich the marks are applied by irradiating a diamond gemstone withultraviolet laser radiation using a projection mask. U.S. Pat. No.4,425,769 describes providing an identifying mark on a diamond or othergemstone by applying a photoresist to the surface, forming a contactmask by a photographic method, and etching the gemstone through the markby cathode bombardment with an ionised gas to provide sputter etching.Sputter etching gives poor control of the depth of the mark and lowresolution.

THE INVENTION

According to a first aspect of the present invention, the surface of adiamond or gemstone is marked with a focused ion beam, the mark beinginvisible to the naked eye. The invention extends to a diamond orgemstone which has been marked by the method of the invention, and toapparatus for carrying out the method.

The marking can be carried out by direct writing on the diamond orgemstone surface with a focused ion beam. Typically Gallium ions areused, but a beam of other suitable ions may alternatively be used. Bylimiting the dose, sputtering of carbon atoms can be substantiallyavoided, sputtering causing direct material removal; this enables a markto be applied with a controlled depth and good resolution. By limitingthe dose, and providing there is sufficient dose, the incident ionscause disordering of the crystal lattice. In the case of diamond, thisconverts the diamond to a graphite-like or other non-diamond structurethat can then be cleaned, e.g. using an acid or potassium nitratedissolved in acid, to leave a shallow mark say not less than 10 nm deepand/or not more than 70 nm deep, more preferably say not less than 20 nmdeep and/or not more than about 50 nm deep, typically about 30 nm deep,with no evidence of blackening. Plasma etching may be used as analternative to acid cleaning.

However, in a preferred embodiment, the disordered layer produced on thediamond or gemstone by the ion beam is removed by means of a powerfuloxidizing agent, such as molten potassium nitrate. This method allows amark to be produced at a lower dose and therefore in less time at agiven beam current. Alternatively, a lower beam current, giving asmaller spot size may be used to produce marks with higher resolutionfeatures, such as diffraction gratings.

The depth of the lattice disordering is determined by the range of theions. For 50 keV Gallium, this range is about 30 nm. The minimum dosemay be as low as 10¹³/cm², but is preferably about 10¹⁴/cm² to 10¹⁵/cm².However, good marks can be applied with a fairly modest dose, thepreferred maximum dose being about 10¹⁶/cm² or even up to about10¹⁷/cm². However, the dose depends upon the ions being used and theirenergy (as measured in keV). The ion beam dose is a total number ofincident ions per unit area at the sample surface, during the marking.The beam current may be about 1 nA, and the beam energy not less thanabout 10 keV or about 30 keV and/or not greater than about 100 keV orabout 50 keV.

It has been found that if depth of mark is plotted against ion beam dosefor a series of different beam energies, there is an increase of depthof mark with increasing beam energy. Characteristics of the mark may beoptimised by selecting from the dose/energy combinations which willresult in the desired depth of mark.

The region to be marked and/or the surrounding area may be coated withan electrically-conducting layer, for instance gold, prior to formingthe mark, so that an electrical connection can be provided beforemarking with the ion beam, to prevent charging. The thickness of thegold, or other, coating alters the variation of depth of mark with beamenergy and dose, and may thus be chosen to optimise the mark produced.

Other suitable methods to reduce charging may be used. One method is toirradiate the region to be marked with a low energy ion beam, e.g. about3 to about 10 keV, prior to forming the mark, to modify the diamondsurface to cause it to become electrically conductive, the electricalconnection being made to that region. In a preferred embodiment, the ionbeam used for marking may be used in conjunction with a chargeneutralising device, such as an electron flood gun, such as thatdescribed in U.S. patent specification number U.S. Pat. No. 4,639,301,to prevent charging of the diamond surface.

In accordance with a second aspect of the present invention, there isprovided a method of marking the surface of a diamond or gemstone,comprising the steps of irradiating at least a portion of said diamondor gemstone to form a damaged or crystal lattice disordered layerthereon, and removing said disordered layer using an oxidizing agent.

A further advantage of the second aspect of the present invention overacid-cleaning is that no acid fumes are produced and also that spentacid does not have to be disposed of, thereby improving the safety ofthe process as well as offering environmental and economic benefits.

The oxidizing agent is preferably molten potassium nitrate. The diamondor gemstone is preferably covered with potassium nitrate and heated to atemperature of around 380-550 Centigrade for a period of between a fewminutes and several hours, preferably approximately one hour.

However, other suitable powerful oxidizing agents include moltencompounds such as alkali metal salts. Suitable compounds may be in theform Xn Ym where the group X may be Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺, or othercation, and the group Y may be OH⁻, NO₃ ⁻, O₂ ²⁻, O²⁻, CO₃ ²⁻, or otheranion; the integers n and m being used to maintain charge balance.Mixtures of compounds may be used. Air or other oxygen-containingcompounds may also be present.

The use of such oxidizing agents to remove a disordered layer allows amark of a desired depth to be produced using a relatively low dose ofions.

In a preferred embodiment, the diamond or gemstone is irradiated with anion beam as in the first aspect of the present invention, and mostpreferably a Gallium ion beam. The preferred embodiment of the method ofthe second aspect resulting in a remarkably efficient process, with eachincident Gallium ion ultimately resulting in the removal ofapproximately 2,700 carbon atoms. In most materials other than diamond,this figure would be around 1-10.

It is this property of diamond that allows the relatively largestructures such as alphanumeric characters covering an area of 0.43 mmby 0.16 mm to be machined in a reasonably economic time of about 10seconds.

The method of the present invention may also be used to mark the surfaceof a synthetic gemstone, such as the silicon carbide gemstones describedin WO 97/09470.

EXAMPLE

A diamond gemstone is mounted in a suitable holder and a facet is coatedwith a layer of gold. The sample is placed in a vacuum chamber equippedwith a focused ion beam source such as supplied by FEI or Micrion, theholder making an electrical connection to the gold layer to prevent thediamond becoming charged. Using a focused beam with a raster scan orsimilar to scan the beam for instance with electrostatic deflection (asan alternative, the diamond may be moved, but this is less practical), amark is written on the diamond facet with ions to a dose of 10¹⁵ to10¹⁶/cm², the ion source being Gallium, the beam current 1 nA and thebeam energy 30 to 50 keV. The sample is removed from the vacuum chamberand acid cleaned to remove the disordered layer and the gold layer.There is a shallow mark typically about 30 nm deep, with no evidence ofblackening.

The present invention has been described above purely by way of example,and modifications can be made within the spirit of the invention, whichextends to the equivalents of the features described. The invention alsoconsists in any individual features described or implicit herein orshown or implicit in the drawings or any combination of any suchfeatures or any generalisation of any such features or combination.

What is claimed is:
 1. A method of forming on a polished facet of adiamond or silicon carbide gemstone an information mark which isinvisible to the eye using a ×10 loupe, comprising: coating at least aportion of said facet with an electrically-conducting layer; forming themark in said facet portion with a focused ion beam while substantiallyavoiding sputtering by moving the beam relative to the gemstone, thebeam energy being from about 10 to about 100 keV, a dose of not lessthan about 10¹³/cm² and of not more than about 10¹⁷/cm² being applied,whereby the beam penetrates the electrically-conducting layer and formsfrom material of the diamond or gemstone a disordered layer in saidfacet portion; and removing said disordered layer by substantiallycovering the disordered layer with molten potassium nitrate, therebyforming a mark whose depth is from about 10 to about 70 nm and whichcomprises at least one line the ratio of the width to depth of which isgreater than about 20:1.
 2. A method of forming in the surface of agemstone an information mark which is invisible to the naked eye andcomprises at least one line having a width and a depth, the ratio ofwhich is greater than about 20:1, by removal of material of saidgemstone, comprising using a focused ion beam while substantiallyavoiding sputtering.
 3. A method of forming in the surface of a diamondan information mark which is invisible to the naked eye and comprises atleast one line having a width and a depth, the ratio of which is greaterthan about 20:1, by removal of material of said diamond, comprisingusing a focused ion beam while substantially avoiding sputtering.
 4. Themethod of claim 2, wherein the gemstone is a silicon carbide gemstone.5. A method of marking the surface of a gemstone, comprising the stepsof irradiating with an ion beam at least a portion of said gemstone toform a mark, comprising a disordered layer therein from material of thegemstone, and removing said disordered layer using an oxidizing agentthereby forming a mark.
 6. The method of claim 5, wherein the gemstoneis a silicon carbide gemstone.
 7. A method of marking the surface of adiamond, comprising the steps of irradiating with an ion beam at least aportion of the diamond to form a mark, comprising a disordered layertherein from material of the diamond, and removing said disordered layerusing an oxidizing agent thereby forming a mark.
 8. The method of claim7, wherein the diamond is irradiated using a focused ion beam.
 9. Themethod of claim 7, wherein the diamond is irradiated using a focused ionbeam whilst substantially avoiding sputtering.
 10. The method of claim2, wherein the surface of the diamond is irradiated by means of saidfocused ion beam to form a disordered layer thereon, and said disorderedlayer is removed using an oxidizing agent.
 11. The method of claim 7,wherein the oxidizing agent is at least one compound in the form XnYmwhere the group X is a cation, and the group Y is an oxygen-providinganion; the integers n and m being used to maintain charge balance. 12.The method of claim 7, wherein the oxidizing agent is potassium nitrate.13. A method of marking the surface of a diamond, comprising forming aninformation mark with a focused ion beam while substantially avoidingsputtering, the mark being invisible to the naked eye, and including thesteps of irradiating at least a portion of the gemstone with an ion beamto form a disordered layer therein and removing said disordered layer bysubstantially covering the disordered layer with molten potassiumnitrate.
 14. A method of marking the surface of a diamond, comprisingforming an information mark with a focused ion beam while substantiallyavoiding sputtering, the mark being invisible to the naked eye, andincluding the steps of irradiating the surface of the diamond by meansof said focused ion beam to form a disordered layer therein, andremoving said disordered layer using an acid.
 15. The method of claim14, wherein said disordered layer is removed using an oxidizing agentdissolved in acid.
 16. The method of claim 15, wherein said disorderedlayer is removed using potassium nitrate dissolved in acid.
 17. Themethod of claim 2, including coating said surface with anelectrically-conductive layer prior to forming the mark.
 18. The methodof claim 1, wherein the electrically-conductive layer is gold.
 19. Themethod of claim 2, wherein the region to be marked is irradiated with alow energy ion beam prior to forming the mark, to modify the diamondsurface to cause it to become electrically conductive.
 20. The method ofclaim 2, wherein the region to be marked is simultaneously irradiatedusing a charge neutralizing device.
 21. The method of claim 2, whereinthe mark is formed at an ion dose of not more than about 10¹⁷/cm². 22.The method of claim 21, wherein the mark is formed at an ion dose of notmore than about 10¹⁶/cm².
 23. The method of claim 21, wherein the markis formed at an ion dose of not more than about 10¹⁵/cm².
 24. The methodof claim 21, wherein the mark is formed at an ion dose of not more thanabout 10¹⁴/cm².
 25. The method of claim 21, wherein the mark is formedat an ion dose of not more than about 10¹³/cm².
 26. The method of claim2, wherein the depth of the mark is about 10 to 70 nm.
 27. The method ofclaim 26, wherein the depth of the mark is about 20 to 50 nm.
 28. Themethod of claim 26, wherein the depth of the mark is about 20 to 30 nm.29. The method of claim 2, wherein the mark comprises characters whoseheight is about 50 microns.
 30. The method of claim 2, wherein the markcomprises lines of a width of about 2 to 3 microns.
 31. The method ofclaim 2, wherein the depth of the mark is not more than about 100 nm.32. The method of claim 2, wherein the mark comprises lines the ratio ofthe width to depth of which is greater than about 20:1.
 33. The methodof claim 2, wherein the mark is an information mark.
 34. The method ofclaim 2, wherein the mark is invisible to the eye using a ×10 loupe. 35.The method of claim 7, wherein the mark is invisible to the naked eye.36. The method of claim 2, wherein the mark is applied to a polishedfacet of a gemstone or diamond.
 37. A gemstone which has in a facetthereof a mark formed by the method of claim 2 by the removal ofmaterial of said gemstone, the mark being an information mark which isinvisible to the naked eye and comprises at least one line having awidth and a depth, the ratio of which is greater than 20:1.
 38. Themethod of claim 1, wherein said dose is of not less than about 10¹⁵/cm².39. The method of claim 2, wherein the focused ion beam is movedrelative to the gemstone, thereby directly writing on the gemstone. 40.The method of claim 11, wherein the group X is selected from the groupconsisting of Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺.
 41. The method of claim 11,wherein Y is selected from the group consisting of OH⁻, NO⁻ ₃, O₂ ²⁻,O²⁻ and CO₃ ²⁻.
 42. The method of claim 40, wherein Y is selected fromthe group consisting of OH⁻, NO⁻ ₃, O₂ ²⁻, O²⁻ and CO₃ ²⁻.
 43. Themethod of claim 5, wherein the mark comprises at least one line theratio of the width to depth of which is greater than about 20:1.
 44. Themethod of claim 7, wherein the mark comprises at least one line theratio of the width to depth of which is greater than about 20:1.
 45. Adiamond which has in a facet thereof a mark formed by the method ofclaim 3 by the removal of material of said diamond, the mark being aninformation mark which is invisible to the naked eye and comprises atleast one line having a width and a depth, the width to depth ratiobeing greater than about 20:1.
 46. The method of claim 9, whereingallium ions are used for the focused ion beam and, after removing thedisordered layer with the oxidizing agent, each incident gallium ionresults in the removal of more than 1000 carbon atoms.
 47. The method ofclaim 9, wherein gallium ions are used for the focused ion beam and,after removing the disordered layer with the oxidizing agent, eachincident gallium ion results in the removal of close to 2700 carbonatoms.
 48. The method of claim 1, wherein the diamond is a gemstone. 49.The method of claim 3, wherein the diamond is a gemstone.
 50. The methodof claim 7, wherein the diamond is a gemstone.
 51. The method of claim5, wherein the gemstone is irradiated whilst substantially avoidingsputtering using a focused ion beam.
 52. The method of claim 5, whereinthe mark is an information mark which is invisible to the naked eye andcomprises at least one line having a width and a depth, the ratio ofwhich is greater than about 20:1.
 53. The method of claim 7, wherein themark is an information mark which is invisible to the naked eye andcomprises at least one line having a width and a depth, the ratio ofwhich is greater than about 20:1.
 54. The method of claim 3, wherein themark is formed at an ion dose of not more than about 10¹⁷/cm².
 55. Themethod of claim 5, wherein the mark is formed at an ion dose of not morethan about 10¹⁷/cm².
 56. The method of claim 7, wherein the mark isformed at an ion dose of not more than about 10¹⁷/cm².
 57. The method ofclaim 3, wherein the depth of the mark is not more than about 100 nm.58. The method of claim 5, wherein the depth of the mark is not morethan about 100 nm.
 59. The method of claim 7, wherein the depth of themark is not more than about 100 nm.
 60. A method of marking the surfaceof a diamond gemstone comprising forming in a polished facet of thegemstone an information mark which is invisible to the naked eye byirradiating the facet with a focused ion beam with an ion dose of notmore than 10¹⁷/cm² in order to form a disordered layer in said facetcorresponding to said mark and removing said disordered layer using anoxidizing agent.
 61. The method of claim 60, wherein the depth of themark is not more than about 100 nm.
 62. The method of claim 60, whereinthe mark comprises at least one line having a width and a depth, theratio of which is greater than about 20:1.
 63. The method of claims 31,wherein the mark is formed at an ion dose of not more than about10¹⁷/cm².
 64. The method of claim 7, wherein the mark is formed at anion dose of not more than about 10¹⁶/cm².
 65. The method of claim 7,wherein the mark is formed at an ion dose of not more than about10¹⁵/cm².
 66. The method of claim 7, wherein the mark is formed at anion dose of not more than about 10¹⁴/cm².
 67. The method of claim 7,wherein the mark is formed at an ion dose of not more than about10¹³/cm².
 68. The method of claim 7, wherein a depth of the mark is lessthan about 70 nm.
 69. The method of claim 7, wherein a depth of the markis less than about 50 nm.
 70. The method of claim 7, wherein a depth ofthe mark is less than about 30 nm.
 71. The method of claim 5, whereinthe mark is formed at an ion dose of not more than about 10¹⁷/cm². 72.The method of claim 7, wherein the mark is formed at an ion dose of notmore than about 10¹⁷/cm².
 73. The method of claim 3, wherein the depthof the mark is not more than about 100 nm.
 74. The method of claim 5,wherein a depth of the mark is not more than about 100 nm.
 75. Themethod of claim 1, wherein the depth of the mark is not more than about70 nm.